PLL heterodyne converters are known and used, for example, in video disc players to facilitate chrominance signal frequency translation and time-base error correction. An example of a player having such a converter is described in U.S. Pat. No. 3,965,482 entitled "VELOCITY CORRECTION CIRCUIT FOR VIDEO DISCS" which issued to T. W. Burrus, June 22, 1976, and is incorporated herein by reference. The Burrus player includes a turntable for rotating a video disc, a pick-up transducer for sensing capacitance variations representative of information recorded on the disc in the "buried subcarrier" (BSC) format and an FM demodulator responsive to signals derived form the pick-up transducer for providing a composite video signal which is time-base corrected and converted from the BSC format the an NTSC format by means of a video converter.
An advantageous feature of the Burrus player is that errors in the disc-pickup relative velocity and errors in the recovered chrominance signal frequency are corrected by means of a two-loop servo system. It includes a color burst keyed phase detector which produces a composite error signal representative of phase and frequency errors between the output of an NTSC reference frequency (3.58 MHz) crystal oscillator and the color burst component of the chrominance output signal produced by the video converter. A loop filter separates the composite error signal into two component error signals. One of the component error signals is applied to a stylus tangential position control transducer (known as an "arm stretcher") to form a principal feedback loop for minimizing velocity errors of the pickup stylus relative to the surface of the disc. By this means, time base errors in both the chrominance and the luminance components of the composite video output signal (due, for example, to disc warpage or eccentricity) are reduced. The other of the component error signals is applied to a voltage controlled oscillator which forms part of a PLL heterodyne converter. The auxiliary feedback loop then formed (which is nested with the principal feedback loop) provides additional reduction of phase and frequency errors of the chrominance component of the composite video output signal.
Various improvements to the burst locked heterodyne converter of Burrus have been proposed. As an example, U.S. Pat. No. 4,247,866 of Wilber et al., entitled "NESTED LOOP VIDEO DISC SERVO SYSTEM" which issued Jan. 27, 1981, and is incorporated herein by reference, shows a converter having an additional feedback loop which reduces the effect of certain transient disturbances of the primary and nested loops. As another example, U.S. Pat. No. 4,286,282 of Christopher et al., entitled "PERIODICALLY BIASED VIDEO DISC PLAYER SERVO SYSTEM" which issued Aug. 25, 1981 and is incorporated herein by reference, shows a converter in which the keyed phase detector error voltage holding capacitor is periodically precharged which has the effect, inter alia, of reducing the phase lock loop stabilization time. Video disc players having color burst locked heterodyne converters of the type described by Burrus and incorporating the improvements proposed by Christopher et al. and Wilber et al. have been produced commercially, for example, by RCA Corporation (e.g., the model SFT 100).